Many of the demonstrators said they were bent on destroying capitalism as they engaged in cat-and-mouse tactics with police on May Day, breaking up when forced to and then gathering again in small groups for renewed confrontation.
An official with the city's ambulance service said five people were treated — two police officers and three other people.
"Two people were overcome by tear gas, while another was in shock," said Bob Lamle.
"Two of these people may have taken part in the protest, while the third was a bystander."
Police spokesman Laurent Gingras said one officer who suffered facial cuts was taken to hospital with injuries that were not considered life-threatening.
The situation had calmed down a few hours after the protest began at 7 p.m. It took police just a few minutes after it started to declare the march illegal because organizers did not give the route beforehand.
At 11:15 p.m., authorities reported that 17 people had been arrested for illegal gathering, while another 10 faced other charges, including mischief, assaulting a police officer or trying to disarm an officer.
Another 57 people, including two minors, were detained for violating municipal bylaws.
One protester was seen smashing the windows of a cruiser with his fists, while Gingras said several cars belonging to police and civilians were vandalized.
The annual protest in Montreal on May 1 coincides with International Workers Day and usually results in clashes between police and demonstrators.
This year was no exception as protesters took over Sainte-Catherine Street, one of the city's busiest thoroughfares, marching in both directions and blocking traffic for many unsuspecting motorists.
Officers on horses could be seen trying to disperse protesters, some of whom carried placards with the slogan No to the Privatization of Quebec.
Many pedestrians, tourists and people sitting on patios were affected by the tear gas.
"With the wind, customers had to rush inside because of the gas," said Gabrielle Prevost, manager of the busy Benelux pub on Sherbrooke Street.
"We had a lot of people outside and the patio emptied pretty quickly. Many were overcome, especially the servers."
Much of the action took place just a few blocks from the Bell Centre, where more than 20,000 fans peacefully streamed out at about 11 p.m. after seeing the Canadiens lose 2-1 in overtime against the Tampa Bay Lightning in Game 1 of their second-round playoff series.
Earlier, a spokesman for Montreal's Anti-Capitalist Convergence said in a statement that "it's capitalism we must destroy."
"It's our dissent, it's our rebellion, and May 1st is our day," Rich Bonemeal said.
"We don't have to ask permission, say please and thank you, or apologize for our presence disrupting the course of capitalist activity. We are in the streets because we want to be disruptive. It's a right you don't ask for, it's a right you have to claim."
The protest was in stark contrast to those held earlier in the day when thousands took to the streets of Montreal and elsewhere in the province in peaceful demonstrations.
Those marches focused primarily on big banks and the Quebec Liberal government's cost-cutting measures.
Alexa Conradi, president of the Quebec Federation of Women, said the Couillard government hasn't taken into account any of the proposals presented by social groups.
"Instead, what it's doing is causing cuts to education...in public services, in health care and in the regions." she said. "A lot of these services are absolutely essential to maintaining a degree of equality in our society."
Conradi added that various groups decided to protest throughout the province because of what she called the government's refusal to listen to them.
The Montreal demonstrations brought together a variety of groups that included unionists, students and social groups.
Early on Friday, an Old Montreal building that houses many financial and trade institutions was targeted, forcing police to secure the area.
Employees had to show their ID passes to get inside the huge building, which is also home to several provincial government departments.
The biggest march, featuring several thousand people, was noisy but orderly. It ended at the Montreal Stock Exchange after helmeted riot-equipped police had followed them along their route.
Joel Pedneault, a spokesman for the coalition that organized the march, said banks were targeted because of their record profits.
"So we can go and get some money there in order to keep the public services we have and make them even better," he said.
Pedneault said actions were organized in most cities in the province, including Quebec City, Sherbrooke and Chicoutimi.
"Our strategy is to affect the economy as much as we can, so in the morning we were blocking the entrance for people going to the National Bank tower.
"The idea is to have an impact in many places, instead of just one place."
Normally, on May Day, there is usually only one big demonstration.
Daniel Boyer, president of the Quebec Federation of Labour, said members wanted to organize protests in their own region rather than having to travel to Montreal to take part in the traditional huge demonstration.
"We said OK, but it had to be sufficiently big enough to have the same impact or even larger than the (usual) protest," he added.